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Ž . Sensors and Actuators 76 1999 266–272 www.elsevier.nlrlocatersna Micromachined resonator for cavitation sensing E. Peiner a, ) , R. Mikuta b , T. Iwert b,1 , H. Fritsch b , P. Hauptmann b , K. Fricke a , A. Schlachetzki a a Institut fur Halbleitertechnik, Technische UniÕersitat Carolo-Wilhelmina zu Braunschweig, P.O. Box 3329, D-38023 Braunschweig, Germany ¨ ¨ b Institut fur Prozeßmeßtechnik und Elektronik, Otto-Õon-Guericke-UniÕersitat Magdeburg, PO Box 4120, D-39016 Magdeburg, Germany ¨ ¨ Received in revised form 14 January 1999; accepted 28 January 1999 Abstract A micromachined spring-mass resonator is investigated with respect to its response on short impacts. The resulting amplitude spectrum is dominated by resonances which are related to bending and torsional vibration modes of the resonator. The amplitude ratio of these modes which depends on the direction of excitation is used to monitor at high sensitivity characteristic parts of complex solid-borne sound spectra of machines. For example, the solid-borne sound emitted by a rolling bearing as well as by a centrifugal pump under different conditions of operation was investigated. The results obtained with the pump show that cavitation can be detected—an effect which may cause severe damage to the pump body due to material erosion. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Micromachined vibration sensor; Resonant operation; Bending and torsional vibration modes; Cavitation sensing 1. Introduction State-dependent maintenance is necessary to minimize down-times and to avoid accidental shutdown of highly productive industrial lines. This requires continuous moni- toring of moving parts of machines with respect to solid- borne vibrations. A monitoring and diagnosis system con- sisting of one or several sensors mounted in close distance to the vibration source are linked to a programmable logic controller by a field bus. Machine vibrations may comprise spectral components in the frequency range of several Hz up to 100 kHz leading to data rates of Mbitrs. Especially for a monitoring system composed of a number of test points the capacity of a field bus may be exceeded. Thus, data reduction before readout is necessary, e.g., by filtering the significant components of a vibration spectrum. Vibration spectra comprising characteristic frequencies are generated during the operation of rolling bearings. These are the so-called kinematic frequencies which are related to periodic impacts induced by the overrolling of ) Corresponding author. Tel.: q49-531-3913761; Fax: q49-531- 3915844; E-mail: [email protected] 1 Present adress: m-Sen Mikrosystemtechnik, Prof.-Hermann-Klare- Strasse 6, D-07407 Rudolstadt, Germany. flaws on the surface of the rolling elements or the race- w x ways of a bearing 1–3 . Fig. 1a shows the top view of a portion of the outer raceway of a rolling bearing mounted in a wheel set of a waggon. On the surface of the race small imperfections are visible. At a velocity of 62.6 kmrh overrolling of these imperfections leads to periodic bursts at a frequency of 56.7 Hz as defined by the geomet- rical dimensions of the bearing. Using a conventional broadband piezoelectric sensor this kinematic frequency cannot be detected directly in the base-band vibration Ž . spectrum Fig. 1b but must be separated from the back- ground by analysis of the envelope of the signal limited to a specially selected frequency band. This procedure re- quires additional effort and experience. However, using a spring-mass resonator operated close to its resonance fre- quency the characteristic vibration peak appears at large Ž . signal-to-noise ratio see Fig. 1c . In this case, condition assessment with the bearing can be performed directly using the time-domain signal and online monitoring is w x possible 4,5 . Vibration control of machines is not restricted to the monitoring of periodic bursts. During operation of pumps or turbines uncorrelated impacts lacking a defined direc- tion of excitation may contribute to the solid-borne sound emission signal. Such impacts are generated, e.g., by cavi- tation, which can cause severe damage to a pump or a 0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0924-4247 99 00048-5

Micromachined resonator for cavitation sensing

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Page 1: Micromachined resonator for cavitation sensing

Ž .Sensors and Actuators 76 1999 266–272www.elsevier.nlrlocatersna

Micromachined resonator for cavitation sensing

E. Peiner a,), R. Mikuta b, T. Iwert b,1, H. Fritsch b, P. Hauptmann b, K. Fricke a,A. Schlachetzki a

a Institut fur Halbleitertechnik, Technische UniÕersitat Carolo-Wilhelmina zu Braunschweig, P.O. Box 3329, D-38023 Braunschweig, Germany¨ ¨b Institut fur Prozeßmeßtechnik und Elektronik, Otto-Õon-Guericke-UniÕersitat Magdeburg, PO Box 4120, D-39016 Magdeburg, Germany¨ ¨

Received in revised form 14 January 1999; accepted 28 January 1999

Abstract

A micromachined spring-mass resonator is investigated with respect to its response on short impacts. The resulting amplitude spectrumis dominated by resonances which are related to bending and torsional vibration modes of the resonator. The amplitude ratio of thesemodes which depends on the direction of excitation is used to monitor at high sensitivity characteristic parts of complex solid-bornesound spectra of machines. For example, the solid-borne sound emitted by a rolling bearing as well as by a centrifugal pump underdifferent conditions of operation was investigated. The results obtained with the pump show that cavitation can be detected—an effectwhich may cause severe damage to the pump body due to material erosion. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: Micromachined vibration sensor; Resonant operation; Bending and torsional vibration modes; Cavitation sensing

1. Introduction

State-dependent maintenance is necessary to minimizedown-times and to avoid accidental shutdown of highlyproductive industrial lines. This requires continuous moni-toring of moving parts of machines with respect to solid-borne vibrations. A monitoring and diagnosis system con-sisting of one or several sensors mounted in close distanceto the vibration source are linked to a programmable logiccontroller by a field bus. Machine vibrations may comprisespectral components in the frequency range of several Hzup to 100 kHz leading to data rates of Mbitrs. Especiallyfor a monitoring system composed of a number of testpoints the capacity of a field bus may be exceeded. Thus,data reduction before readout is necessary, e.g., by filteringthe significant components of a vibration spectrum.

Vibration spectra comprising characteristic frequenciesare generated during the operation of rolling bearings.These are the so-called kinematic frequencies which arerelated to periodic impacts induced by the overrolling of

) Corresponding author. Tel.: q49-531-3913761; Fax: q49-531-3915844; E-mail: [email protected]

1 Present adress: m-Sen Mikrosystemtechnik, Prof.-Hermann-Klare-Strasse 6, D-07407 Rudolstadt, Germany.

flaws on the surface of the rolling elements or the race-w xways of a bearing 1–3 . Fig. 1a shows the top view of a

portion of the outer raceway of a rolling bearing mountedin a wheel set of a waggon. On the surface of the racesmall imperfections are visible. At a velocity of 62.6kmrh overrolling of these imperfections leads to periodicbursts at a frequency of 56.7 Hz as defined by the geomet-rical dimensions of the bearing. Using a conventionalbroadband piezoelectric sensor this kinematic frequencycannot be detected directly in the base-band vibration

Ž .spectrum Fig. 1b but must be separated from the back-ground by analysis of the envelope of the signal limited toa specially selected frequency band. This procedure re-quires additional effort and experience. However, using aspring-mass resonator operated close to its resonance fre-quency the characteristic vibration peak appears at large

Ž .signal-to-noise ratio see Fig. 1c . In this case, conditionassessment with the bearing can be performed directlyusing the time-domain signal and online monitoring is

w xpossible 4,5 .Vibration control of machines is not restricted to the

monitoring of periodic bursts. During operation of pumpsor turbines uncorrelated impacts lacking a defined direc-tion of excitation may contribute to the solid-borne soundemission signal. Such impacts are generated, e.g., by cavi-tation, which can cause severe damage to a pump or a

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0924-4247 99 00048-5

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( )E. Peiner et al.rSensors and Actuators 76 1999 266–272 267

Ž .Fig. 1. Outer race of a rolling bearing a showing small imperfections onits surface and vibration spectra measured during operation at a rotational

y1 Ž .speed of 332 min by a commercial broadband piezoelectric sensor bŽ .as well as by a micromachined resonator c .

turbine by large-area material erosion. In this study theresponse of micromachined resonators on uncorrelated im-pacts of undirected excitation is described. In this contextcavitation of a centrifugal pump is investigated.

2. Sensor construction and fabrication

2.1. Micromachined resonator

Fig. 2 shows a schematic of a spring-mass resonatorfabricated using silicon bulk micromaching techniquesw x6,7 . It consists of a seismic mass supported by a thinsuspension as the spring. The stress generated at the springsupport is converted into an electrical signal by a symmet-ric Wheatstone bridge using the piezoresistive effect offour p-type resistors. The bridge signal is amplified on-chip

Fig. 2. Schematic of the spring-mass resonator.

Ž .using a metal-oxide-semiconductor MOS circuit. Theamplifier output modulates the drive current of a hybrid

Ž .AlGaAsrGaAs light-emitting diode LED and thus itsoptical output power. The wavelength of the emitted light,which is launched into a standard multimode fibre withcore and cladding diameters of 50 and 125 mm, respec-tively, is 870 nm. Finally, the optical signal is acquired bya Si photodiode. This fibre-optical signal readout providesseveral benefits, e.g., immunity against electromagneticinterferences which can be a severe problem in the envi-

w xronment of industrial production 4,5 .The vibration sensor is realized by bulk micromachin-

w xing in 100 -oriented, n-type Si of a specific resistivity of2–4 V cm. The basic steps of the fabrication process areschematically displayed in Fig. 3. The process starts with

Ž .backside etching of the Si wafer using KOH 30 wro atŽ .608C upper schematic to predefine the resonator structure

and a groove employed as alignment aid for an opticalfibre.

Subsequently, boron diffusion at 11008C from a silicaemulsion is performed to realize the piezoresistors, and thesourcerdrain regions of MOSFETs. This step is followed

Ž .by thermal oxidation at 11008C field and the gate oxides .Ž . Ž .As the metallization a Au 300 nm rCr 30 nm bilayer is

evaporated which has a very low etching rate in KOH.This is important for the subsequent structuring of thesensor element. The resulting structure is displayed in Fig.3b.

Fig. 3. Fabrication steps of the micromachined resonator in a schematicrepresentation.

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Table 1Typical characteristics of realized prototypes

Parameter Value

Ž .Resonance frequency f 100 Hz0Ž .Sensitivity 10 mVr V g at Wheatstone bridgeŽ .1 mVr V g at Photodetector

Resolution -5 mg at Wheatstone bridge-100 mg at Photodetector

Range "4 g at resonant excitation"70 g at shock excitation of 1 ms duration

In the course of the final structuring the thickness of thespring is precisely defined and a via hole for fibre-to-LEDcoupling is opened. The LED is positioned by a microgrip-per onto the via hole where it is fixed by conductive epoxyresin. To improve fixing and for insulation transparentepoxy resin is used. Finally, conductive epoxy resin isdeposited for electrical contact to the top electrode. Passivealignment by the inclined sidewalls of the opening is usedto position the optical fibre with respect to the LED.Transparent epoxy resin is utilized for fixing. A schematiccross-section of the complete sensor is displayed in Fig.3c. The characteristic data of realized sensor prototypes aresummarized in Table 1.

2.2. Signal conditioning and eÕaluation unit

For spectrum analysis the sensor signal can be acquiredat a sampling rate of 400 kHz and a resolution of 16 bit by

Fig. 4. Photograph of the realized signal conditioning and evaluation unit.

a PC equipped with a signal analyser unit. For onlinevibration monitoring applications, the sensor is connectedto an analog signal conditioning circuit for amplification

Ž .and rectification gain 46 to 50 dB, time constant 1 to 3 s .Ž .After analog-to-digital conversion 8 bit the signal is

Žanalysed using a programmable microcontroller Motorola.68HC11E2 . Additional input ports are available for refer-

ence signals like broadband solid-borne sound, temperatureor rotational speed of the investigated machine. For signalevaluation a fuzzy algorithm is implemented using FUDGEV1.02. A serial link inputroutput unit connected to a

Ž .controller area network CAN driver is employed forsignal output to a PC via a twisted pair cable. There aCAN controller unit serves as the bus master. An impor-tant advantage of this type of field bus is the possible

Žconnection of a number of sensors up to 16 in our.configuration without additional effort for wiring. On the

PC the evaluated signal can be visualized, logged andstored for further analysis. The complete signal condition-ing and evaluation unit fabricated on an Al O substrate2 3Ž .Hoechst CeramTec is shown in Fig. 4. The strip conduc-tors are realized by screen printing. For the passive compo-nents of the analog and digital units surface mounting andscreen printing are employed, respectively.

3. Spring-mass resonator

3.1. Excitation by an impact

Excitation of the mass m of a spring-mass resonator bya short impact pulse maD t of an amplitude a and aduration D t at the time ts t leads to an exponentially0

Ž . w xdecaying oscillation d t 8 :

aD t v0d t s exp y ty t sin v ty t 1Ž . Ž . Ž . Ž .0 0 0

v 2Q0

with the resonance frequency v s2p f , the quality fac-0 0Ž .tor Qsv mrb Q41 and the damping coefficient b.0

In Fig. 5, the experimentally found Fourier spectrum of theresponse of a spring-mass resonator excited by a shock

Fig. 5. Response of a micromachined resonator on shock excitation.

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impulse of 23.6 g at a duration of 1 ms along its mostŽ .sensitive axis, i.e., the z-axis cf. upper part of figure is

Ž .displayed. As expected according to Eq. 1 the spectrumis dominated by a peak at the resonance frequency of theresonator. First and second higher harmonics are dampedby 45 and 65 dB, respectively.

3.2. Periodic excitation

Fig. 6 shows the amplitude spectrum of a microma-chined resonator excited at frequencies ranging from 0 to1.2 kHz by a periodic acceleration a along its mostsensitive axis. Parameter to the curves is the temperature.The resonances at f s86 Hz, f s0.53 kHz and f s1.10 t 1

kHz correspond to the fundamental bending vibration, thetorsional mode and the first higher bending mode of theresonator, respectively. We identified these modes by fi-

Ž .nite-element-model FEM calculations based on the knowndimensions of the resonator. The appearance of the tor-sional mode which is not expected according to the simula-tion given ideal geometric dimensions can be assigned to aslight misalignment of the resonator to the excitation axis.We note the remarkably small temperature sensitivity.

3.3. Dependence on excitation direction

ŽA short impact along its most sensitive axis z-axis, see.Fig. 5 causes a spring-mass system to vibrate in its

fundamental bending mode. By FEM we find that thetorsional and the first higher bending mode are excitedonly to 1% and 2%, respectively, of the entire vibrationenergy. However, their contributions to the vibration signalconsiderably increase when the excitation direction devi-ates from the most sensitive axis of the sensor.

Ž .In Fig. 7 the amplitudes of the fundamental mode aŽ .and the torsional mode b of the sensor on impact pulses

of 2.4 ms duration are displayed in dependence on the tiltŽ .of the resonator about its main axis x-axis . As shown in

the upper part of this figure the resonator has a masssuspended by two cantilevers. At 08 corresponding to an

Fig. 6. Amplitude spectrum of a micromachined resonator on periodicŽ .excitation showing resonances due to fundamental bending f , torsional0

Ž . Ž .f and first higher bending mode f .t 1

Ž .Fig. 7. Amplitudes of the fundamental mode a and the torsional modeŽ .b of a micromachined resonator in dependence on the excitation direc-tion.

excitation along the z-axis the contribution of the torsionalmode is between 0.2 and 0.8% which increases withincreasing tilt. The fundamental mode reaches a minimumvalue of its amplitude at 908 while the torsional mode ismost effectively excited at 758. The maximum amplituderatio of the torsional mode to the fundamental mode is 35to 45%. This result is in reasonable agreement with theFEM calculation by which 48% is found. A higher sensi-tivity with respect to the excitation of the torsional modeleading to an amplitude ratio of around 100% is foundexperimentally by modifying the design of the resonatorspring, i.e., the distance between the two cantilevers isreduced by a factor of 2.

4. Vibration measurements

Using the realized micromachined resonators we inves-Ž .tigated damage of a deep groove ball bearing FAG 6209

Žand a centrifugal pump Etaline 100-125, KSB Franken-.thal, Germany with respect to cavitation. In the first case

the direction along which vibrations are excited is animportant parameter for diagnosis. We positioned the sen-

Ž .sor with its most sensitive axis z-axis along the radialŽ . Ž .position I and the axial symmetry axis position II of thebearing, respectively. Vibration measurements were per-formed with an undamaged bearing and a bearing contain-ing a defect on the surface of its inner race. In theamplitude spectra fundamental bending, torsional and firsthigher bending modes appeared indicated by peaks at f ,0

f and f . For the sensor at position I the damaged bearingt 1

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( )E. Peiner et al.rSensors and Actuators 76 1999 266–272270

showed an increase of the amplitudes of these peaks by 22to 25 dB with respect to the undamaged reference. Theamplitude ratio of the torsional mode to the fundamentalbending mode was 1.5. With the sensor at position II theamplitude of f was increased by 44 dB while for f the0 t

increase amounted only to 12 dB. In this case the ampli-tude ratio of torsional to fundamental mode was only 6%.Based on the measured dependence of the sensor response

Ž .on excitation direction cf. Fig. 7 we can conclude that theinvestigated inner-race damage preferentially led to axialvibrations of the ball bearing.

Furthermore, we used the described spring-mass res-onator to investigate the influence of cavitation on thecomplex spectra of the solid-borne sound emitted by acentrifugal pump. The micromachined resonator and a

Ž .broadband reference sensor 100 mVrg were positionedat the intake of the pump. Cavitation occurs when particlesof an incompressible fluid get into a region of local lowpressure, i.e., where the static pressure is below the vapourpressure of the fluid. In this case vapour bubbles aregenerated which then move by the mass flow into a regionwhere the static pressure is higher than the vapour pres-sure. There the bubbles instantaneously collapse generatinghighly energetic fluidic beams. When striking the surfaceof the pump body material erosion can take place indicatedby short impact pulses in the solid-borne vibration signal.

In the first series of measurements we increased therotational speed of the pump from 1500 to 3000 miny1

leaving the mass flow cross-section constant. At 1500miny1 where cavitation did not occur the signal amplitudeof the broadband sensor is below 25 mV. Incipient cavita-tion was detected at 2000 miny1 by broad vibration bands

Ž .around 2.5 and 4.3 kHz 70 mV . After further increase of

the rotational speed to 3000 miny1 the amplitude of the4.3 kHz band increased to 140 mV. Similarly, cavitationwas found leaving the rotational speed constant but reduc-ing the mass-flow cross-section. Fig. 8 shows the outputsignal and its Fourier spectrum measured by the broadbandsensor at a rotational speed of 2000 miny1. Cavitation is

Žindicated by strong pulses in the time-domain signal Fig..8b which are not detected during cavitation-free operation

Ž . Ž .Fig. 8a . In the corresponding spectra Fig. 8c and dcavitation is related to an increase of broad bands between2.5 and 7.5 kHz.

For comparison, Fig. 9 shows the vibration signal mea-sured using a micromachined resonator at 3000 miny1.Under normal operation conditions both the output signalŽ . Ž .Fig. 9a and the amplitude spectrum Fig. 9c indicate bythe dominant appearance of the fundamental mode that the

Žexcitation is essentially along the z-axis of the sensor cf..Section 3.3 . At incipient cavitation torsional and first

higher bending mode increase at the expense of the funda-Ž .mental mode Fig. 9b and d . Following the measured

Ž .dependence on excitation direction Fig. 7 the increasedamplitude of the torsional vibration as well as the suppres-sion of the fundamental bending mode can be assigned tovibrations of the pump body perpendicular to the z-axis ofthe resonator. These impacts originate from the arbitrarilydirected highly energetic fluidic beams generated duringcavitation.

The results displayed in Figs. 8 and 9 show that thesolid-borne sound emitted by a pump can be used to detectcavitation. Utilizing a commercial broadband sensor ex-tended parts of the baseband signal contain the characteris-

Ž .tic information and must be analysed Fig. 8 . In the caseof a spring-mass resonator, however, sharp vibration modes

Fig. 8. Solid-borne sound and corresponding Fourier spectra measured by a commercial broadband sensor with a centrifugal pump under normal operationŽ . Ž .conditions a,c and at cavitation b,d .

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Ž .Fig. 9. Response of a micromachined resonator on the solid-borne sound emitted by a centrifugal pump under normal operation conditions a,c and atŽ .cavitation b,d .

are excited which indicate cavitation directly by the ratioŽ .of their amplitudes Fig. 9 . Thus, fast and efficient moni-

toring of cavitation is possible at low effort for signalconditioning and analysis.

The selective excitation of sharp vibration modes can beexploited for online monitoring of the operation conditionsof rolling bearings as well as of pumps and turbines withrespect to cavitation. After band-pass filtering of the fun-damental bending mode and the torsion mode their ampli-tude ratio can be used as the input for the described signal

Ž .conditioning and evaluation unit Section 2.2 . Thus, usinga micromachined resonator for vibration measurement theamount of data in the complex solid-borne sound signalemitted by machines can be efficiently compressed to thesignificant information.

5. Conclusions

A spring-mass resonator fabricated by Si micromachin-ing and its response on short impacts is described. Themeasured amplitude spectrum is dominated by vibrationmodes corresponding to bending and torsion of the spring.The ratio of the respective amplitudes depends on thedirection of the exciting impacts. This effect is used tomonitor damage of a rolling bearing and the occurrence ofcavitation during operation of a centrifugal pump. Theobtained results show that by a micromachined resonatorcomplex solid-borne spectra observed by broadband analy-ses can be effectively compressed to the significant infor-mation. Thus the rate capacity requirements for the subse-quent data transfer can be considerably reduced to therange easily provided by a conventional field bus.

Acknowledgements

We are grateful to D. Rummler, C. Pabsch and T.¨Geuer for active technical support during sensor fabrica-tion and characterization. This work was initiated by theGerman Research Society for the Application of Micro-

Ž .electronics DFAM and financially supported by the Ger-Žman Federal Ministry of Economy BMWi, AiF-Nr.

.10764B .

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Erwin Peiner, born in Bad Munstereifel, Germany, in 1960, received the¨Diplom-Physiker and PhD degrees, both in applied nuclear physics, fromthe University of Bonn, Germany in 1985 and 1988, respectively. In 1989he joined the Institut fur Halbleitertechnik of the Technical University¨Braunschweig, where his main field of interest is the integration ofoptoelectronic devices based on IIIrV semiconductors and silicon-basedmicrosystem components.

In 1989 Reinhard Mikuta joined the Institute for Measuring Technologiesand Electronics. He received his PhD degree in 1984 from the TechnicalUniversity Ilmenau, Germany for research on force sensors. The aims ofhis research work are now optimization and application of micromechani-cal vibration sensors for different types of machines.

Thomas Iwert studied physics in Magdeburg. In 1992 he graduated onaxial emission ionization gauges. Since 1996 he has been working as aresearch associate on a project to investigate micromechanical vibrationsensors. Currently, his interests are in the development of vacuum gaugesusing micromachining techniques.

Holger Fritsch studied physics in Magdeburg. In 1993 he graduated onX-ray detectors based on amorphous silicon layers. Since 1993 he hasbeen working as a research associate. His main activities are in the designof micromechanical vibration sensors using FEM simulations.

Peter Hauptmann was born in 1944. He is head of the sensors andmicrosystems group at the Otto-von-Guericke University Magdeburg,Germany. From 1968 to 1985 he worked at the Technical CollegeLeuna-Merseburg, Germany as a lecturer and senior lecturer. In 1973 hereceived the PhD degree from the same institution for a thesis on polymerphysics. He joined the Otto-von-Guericke University Magdeburg, Ger-many in 1985, bringing with him great experience in sensor developmentand application, and is now a professor. He is the author or co-author of115 papers and five books.

Klaus Fricke was born in Soltau, Germany, in 1970. He received hisDiplom-Physiker degree from the Technical University Braunschweig in1996. Currently, he is pursuing his Dr.-Ing degree in the field of Simicromachining and its combination with MOS technology.

Andreas Schlachetzki was born in Breslau, Germany, in 1938. He re-ceived the Diplom-Physiker and PhD degrees, both from the Universityof Cologne, Cologne, Germany, in 1964 and 1969, respectively. While atthe University of Cologne he worked in ferromagnetism and ultrasonics.From 1970 to 1971, he investigated the magnetic properties of rare-earth-hydroxide single crystals at He temperatures at Becton Center, YaleUniversity, New Haven, CT. In 1971, he joined the Research Institute ofthe German Post Office, Darmstadt, Germany, where he was involved inthe epitaxial growth of GaAs, and in basic aspects of fast digital circuitsutilizing the Gunn effect in GaAs. In 1975 he worked for 6 months at theElectrical Communication Laboratory of Nippon Telegraph and Tele-phone, Tokyo. From 1976 to 1984 he was a professor at the TechnicalUniversity Braunschweig, where he worked on the growth of InGaAsPand its use for integrated optical receivers. From 1984 to 1987 he was aprofessor at the Technical University Berlin and at the same time head ofthe Division Integrated Optics at the Heinrich–Hertz-Institut fur¨Nachrichtentechnik, Berlin. Since 1987 he has been the head of theInstitut fur Halbleitertechnik of the Technical University Braunschweig¨where his main activities are in the field of monolithically integratedIIIrV components on Si substrates.